1. Field of the Invention
The present invention relates to an ion generator which generates air ion clusters and an ionizer which removes static electricity on an object surface, and more specifically, to an ion generator and an ionizer having regulated design ranges of voltage, frequency, and airflow rate for efficient generation and conveyance of ions by a plate-like ion generating element.
2. Description of the Background Art
Among conventional general ion generators and ionizers, for example, in the case of a conventional ionizer, corona discharge is caused by applying a high voltage by a high-voltage power supply to a pointed-needle-like ion generating electrode, whereby the air is ionized (for example, refer to Patent Document 1). The needle-like ion generating electrode must form a local electric field at its tip end portion for forming plasma according to air ionization, and its voltage is 5 to 7 kV or more in the case of using a DC high voltage power supply. Therefore, to generate a sufficient amount of ions by applying a DC voltage to the needle electrode, much power is consumed, and the power supply unit inevitably increases in size.
Some of the power supplies for applying a high voltage to be applied to the needle-like ion generating electrode use a piezoelectric transformer for the purpose of downsizing of the device and control of the ion balance, and the voltage thereof is about 2 to 3 kV, and the frequency is 20 to 100 kHz (refer to Patent Document 2). In other words, to downsize the piezoelectric element while securing stable ion generation by using the needle electrode, a high frequency of not less than 20 kHz is inevitably used. However, with this high frequency, a large amount of ozone is produced as a by-product, and due to heating, stable operation is difficult.
In a conventional needle-like ion generating electrode, influences from dust accumulation and wearing due to physical sputtering make it difficult to cause corona discharge, and the ion generation efficiency tends to be lowered. Also, in a ground electrode which is opposed to the needle-like ion generating electrode and provided for stabilizing discharge, due to electrostatic adsorption according to a high voltage and physical sputtering of the ion generating electrode, dust accumulates and the surface is contaminated, and these deteriorate the ion generation efficiency.
Therefore, a user must periodically perform maintenance for improving the ion generation efficiency by cleaning or replacing the pointed portion of the needle-like ion generating electrode and cleaning the ground electrode and the surroundings thereof. Such maintenance involves cleaning of the inside of a structure having the pointed portion, and as a high voltage is applied to this portion, the maintenance is dangerous and troublesome. An ionizer which includes a mechanism for cleaning has been also developed, however, this requires a complicated mechanism (refer to Patent Document 3).
On the other hand, an ion generating element (refer to Patent Documents 4 through 12) and ionizer which use a dielectric body having a discharge electrode and an induction electrode arranged on the surface as ion generating elements and are in plate shapes instead of needle shapes, has been developed (refer to Patent Document 11).
In the techniques shown in Patent Documents 4 through 11, local discharge is caused by applying a high voltage between a discharge electrode and an induction electrode via a dielectric body to generate ions, so that the element is in a flat shape that does not have a physically pointed structure. By providing a fine protrusion pattern on the discharge electrode, it becomes possible to generate both positive and negative ions stably by electric field concentration, and in comparison with the needle-like ion generating electrode, the same ion amount can be generated with a lower voltage and lower power consumption. Further, by providing a plurality of protrusions in the discharge electrode longitudinal direction, ions that are uniform in concentration can be one-dimensionally generated, so that the spatial variation of ion concentration of the needle-like ion generating electrode is reduced.
The element type electrode can generate ions at a low voltage, so that the electrode is hardly deteriorated and dust, etc., hardly adhere thereto, and in addition, ions are generated on the entire surface of the element, so that dust adhesion is dispersed, and as a result, the electrode is hardly contaminated. Even if it is contaminated, it does not have a pointed structure so that cleaning is easy, and, in comparison with the conventional needle-like electrode, maintenance performance is dramatically improved.
In the case where positive ions and negative ions are generated from an ion generating element by applying a high voltage to the above-described electrode structure formed via a dielectric body, a first problem is that the ion balance is lost due to disturbance factors, that is, deterioration of the electrode and efficiency lowering of the power supply. Herein, it is known that the ion concentration change in the atmosphere is expressed as in the following Equation 1 (for example, refer to “Relationship between ion life and aerosol concentration,” Appendix 4, JIS B9929:2006).
                                          ⅆ                          C              ion                                            ⅆ            t                          =                  q          -                      α            ⁢                                                  ⁢                          C              ion                        ⁢                          C              ion                                -                      β            ⁢                                                  ⁢                          C              ion                        ⁢                          C              aerosol                                                          Equation        ⁢                                  ⁢        1                            Cion: positive or negative ion concentration        t: time        q: rate of ion generation        a: positive-negative ion recombination coefficient        b: attachment coefficient of ion to aerosol        Caerosol: aerosol concentration        
Herein, when assuming that the third term of the right-hand side of Equation (1) can be ignored since the aerosol concentration is sufficiently low in a clean room or general manufacturing and indoor conditions, from Equation (1), the ion concentration change is balanced with the rate of ion generation by recombination of positive-negative ions.
In other words, when the rate of ion generation q increases, the ion concentration increases and the recombination rate of the second term of the right-hand side also increases to right the balance, so that a balanced value exists from which the ion concentration does not increase more. Therefore, at a generation rate equal to or more than this balanced condition, in principle, ions can be stably generated even if the ion generation amount slightly changes.
To obtain such stable ion generation, evaluation of the rate of ion generation q is important, however, it is difficult to obtain this theoretically. Therefore, conventionally, the voltage and frequency, etc., to be applied to the element-like electrode were arbitrarily determined by using a general-purpose power supply. For example, in Patent Documents 4 through 8, a continuous wave of a high voltage was applied, and for example, in Patent Document 4, an AC voltage of 2.5 kvp-p and a frequency of 50 kHz were determined, in Patent Document 5, a voltage of 1.9 to 3.25 kvp-p and a frequency of 5 kHz were determined, in Patent Document 6, a voltage of 2.6 to 6.7 kvp-p and a frequency of 40 kHz were determined, in Patent Document 7, a frequency of 100 to 900 Hz was determined, and in Patent Document 8, a voltage of 2 to 4 kVp-p and a frequency of 40 kHz were determined. These could not always realize stable ion generation.
Another problem in the ion generating element using the element-like electrode is that ozone is produced due to creeping discharge when a high-frequency high voltage is applied. That is, oxygen in the air is combined with atomic oxygen by plasma reaction on the creepage surface of the electrode to produce ozone. As an upper limit of the ozone concentration in the air, an allowable concentration of not more than 100 ppb in a working space is regulated (Japan Society for Occupational Health, Maximum Allowable Concentration Commission), and according to the humidity conditions and other operating conditions, 50 ppb, half the allowable concentration, is recommended. Conventionally, attempts were made to lower the ozone concentration by using a high voltage of an intermittent wave or pulsed wave which produces less ozone unlike the continuous wave (Patent Documents 9 through 12). However, in the case of pulse, generally, a power supply to which DC bias components are applied is used, so that, to separately generate positive and negative ions, at least one of each power supply is required, and downsizing of the device is difficult, and ion balance control is difficult. Specifically, the condition of Equation (1) is not spatially uniform, so that it is difficult to secure a stable ion balance. As partially described in Patent Document 12, there is an example in which the positive and negative ions are simultaneously generated by thinning out the sine waves of 100 Hz to about 60 Hz, however, this value is not always suitable for stable ion generation.
On the other hand, when a plate-like ion generating element is combined with an airflow generating mechanism such as a fan and used as an ionizer, improvement in ionization performance has been demanded. According to the EOS/ESD standards as international standards for ionizers, it is regulated that, when a plate monitor (metal plate) with a size of 150 mm×150 mm is charged, the time necessary to ionize it from a potential of 1000 V to 100 V is an ionization characteristic time tn. Industrially, an ionization rate at which tn is 2 seconds or less at a distance of 30 cm is desirable, and this means that an object like a belt conveyor which operates at a rate of about 7.5 cm/sec can be continuously ionized. Herein, it is known that the ionization characteristic time is obtained from Equation (2), as a function of ion concentration and ion flow rate (for example, Equation (2) is true if Equation (4) described in J. M. Crowley, D. Leri, G. Dahlhoff and L. Vevit, J., Electrostatics., 61, p. 71-83 (2004) is simplified by assuming that ion current does not depend on an electric field).
                              t          n                =                              C            ⁢                                                  ⁢            Δ            ⁢                                                  ⁢            V                                              uAC              ion                        ⁢            e                                              Equation        ⁢                                  ⁢        2                            C: capacitance of ionization target        ΔV: potential difference of ionization target        u: ion velocity        A: area of ionization target        e: elementary charge (=1.6×10−9)        
Most of the conventional plate-like ion generating elements do not aim at ionization of object surfaces, so that ion generation conditions are not optimized. Therefore, it was difficult to obtain high-speed ionization performance with which tn of Equation (2) is 2 seconds or less.
Patent Document 1: Japan Patent Pre-Publication No. H04-94099
Patent Document 2: Domestic Re-publication of PCT International Publication No. WO2004/109875
Patent Document 3: Japan Patent Pre-Publication No. 2004-234972
Patent Document 4: Japan Patent Pre-Publication No. H08-82980
Patent Document 5: Japan Patent Pre-Publication No. 2002-365887
Patent Document 6: Japan Patent Pre-Publication No. 2004-103257
Patent Document 7: Japan Patent Pre-Publication No. 2005-328904
Patent Document 8: Japan Patent Pre-Publication No. 2006-222019
Patent Document 9: Japan Patent Pre-Publication 2003-249327
Patent Document 10: Japan Patent Pre-Publication No. 2006-59711
Patent Document 11: Japan Patent Pre-Publication No. 2006-196291
Patent Document 12: Japan Patent Pre-Publication No. 2006-260963